1. Field of the Invention
The present invention relates to image data processing, and more particularly, to a method for loop-filtering image data to reduce a quantization effect and an apparatus therefor.
2. Description of the Related Art
Generally, picture encoding standards such as MPEG of the International Standardization Organization (ISO) and H.263 recommended by the International Telecommunication Union (ITU) adopt block-based motion estimation and discrete cosine transform (DCT) blocks. Most picture encoding standards utilize the DCT in 8×8 pixel block units in order to pack information with a small number of transform coefficients. This block-based DCT scheme is based on the local spatial correlation properties of an image.
However, when the block-based image data is restored, considerable image deterioration, for example, blocking artifacts near the block boundary, corner outliers at the cross points of blocks, and ringing noise near the image edges, occurs. This is because the image data go through the DCT in 8×8 pixel block units prior to quantization. Such deterioration is serious when the image is highly compressed.
Blocking artifacts are caused by grid noise generated along the block boundary in a relatively homogeneous area. The grid noise shows traces of the block-based process at the edges between blocks when the compressed data is displayed on a screen after being restored. Thus, the edges between blocks are noticeable. The corner outlier is generated where the corners of the 8×8 blocks meet. Also, the ringing noise is a typical Gibb's phenomenon occurring by truncation when the coefficients of high-frequency components of the DCT are quantized so as to highly compress the image. Due to the ringing noise, one can notice overlapping of images with a predetermined interval.
In order to reduce the blocking artifacts, the corner outliers and the ringing noise occurring when block-based coding is performed, several methods have been suggested. A single-adaptive filtering (SAF) method has been introduced in order to reduce the quantization effect of JPEG-decompressed images by Y. L. Lee, H. C. Kim, and H. W. Park (“Blocking Effect Reduction of JPEG images by Signal Adaptive Filtering” in Press IEEE Trans. On Image Processing, 1997]. B. Ramanurthi and A. Gersho have suggested a method in which a two-dimensional (2-D) filter is used to reduce the blocking artifact while a one-dimensional (1-D) filter is used to reduce staircase noise [“Nonlinear Space Variant Postprocessing of Block Coded Images”, IEEE Trans. on ASSP, Vol. 34, No. 5, pp 1258-1267, 1986]. Also, an iterative image-recovery algorithm using the theory of projections onto convex sets (POCS) has been proposed by Y. Ynag, N. Galatsanos and A. Katsaggelos [“Projection-Based Spatially Adaptive Reconstruction of Block-Transform Compressed Images,” IEEE Trans. on Image Processing, Vol. 4., No. 7, pp 896-908, July 1995]. Also, an article entitled “A Deblocking Algorithm for JPEG compressed Images Using Overcomplete Wavelet Representations” has been disclosed by Z. Xiong, M. T. Orchard and Y. Q. Zhang [IEEE Trans. Circuits System Video Technology, Vol. 7, No. 2, pp 433-437, 1997].
However, the above methods require complicated computations.